The present invention relates to radial flow lasers and more particularly to an optical resonator capable of producing an output beam having both diameter and power level capable of being varied.
Laser systems which provide an output beam at a very high power level such as those made possible with a combustion driven chemical laser are subject to various restraints which limit the beam quality and output power from such a system. One of these limitations is due to a phenomenon which is commonly referred to as superfluorescene, a condition under which a gain medium having a population inversion produces spurious beams of laser radiation that do not interact with any of the optical components of the resonator. The spurious beams are undesirable because they deplete the population inversion which would otherwise be available for controlled stimulated emission from the working medium.
One of the concept advances to increase the amount of working medium available without incurring superfluoresence effects is disclosed by Freiberg et al in the U.S. Pat. No. 3,969,687 entitled "Standing Wave Unstable Resonators for Radial Flow Lasers" filed on Apr. 14, 1975 and held with the present application by a common assignee. The unstable resonator is formed having a centerline axis with a gain region in the configuration of a thin wall cylinder disposed symmetrically about the centerline axis. A cylindrical gas source region is disposed adjacent to the interior of the gain region and symmetrically about the centerline axis. The system optics form an unstable resonator capable of providing an output beam having an annular cross-section. The resonator is a standing wave positive branch confocal device in which toroidal and annular mirrors are utilized.
The complex optical reflecting surfaces utilized with chemical lasers are extremely difficult and costly to manufacture, especially for high power applications for which water-cooling is required. A stable optical resonator employing simple optical reflecting surfaces and capable of being used in chemical lasers is disclosed by Sziklas in U.S. Pat. application Ser. No. 789,751 entitled "Stable Optical Resonator" filed on Apr. 21, 1977 and held with the present application by a common assignee. A stable resonator is provided which is disposed symmetrically about a centerline axis, having an optical cavity defined at one end by an unstable resonator and at the other end by a reflector. The unstable resonator comprises a convex mirror and a concave mirror. A flat reflector in optical communication with the unstable resonator is capable of retroreflecting the output of the unstable resonator back into the unstable resonator. When an active medium is disposed between the unstable resonator and the reflector, the resonator is capable of providing an output beam having high power and high optical quality. High optical quality of the beam results from the effective discrimination against high order transverse modes provided by the unstable resonator.
A resonator having coupled cavities is disclosed by Sziklas in application Ser. No. 848,697 entitled "Resonator Having Coupled Cavities With Intercavity Beam Expansion Elements" filed on Nov. 4, 1977 and held with the present application by a common assignee. The resonator described therein includes a mode control cavity, a power extraction cavity, a stripper mirror for out coupling a laser beam from the resonator and a conical mirror capable of varying the diameter of a beam of radiation circulating within the resonator and of optically coupling the cavities. In one embodiment the apex of the conical mirror has a convex reflective surface which defines one end of the mode control cavity. The laser beam produced by the resonator has an annular cross-section.
Freiberg in U.S. Pat. No. 4,025,172 entitled "Compound Unstable Resonator" filed Oct. 9, 1975 and held with the present application by a common assignee discloses an unstable resonator having an annular end mirror, a circular end mirror and a conical folding mirror disposed symmetrically about a centerline axis through the resonator wherein the apex of the conical mirror and the center of the circular mirror have reflective surfaces modified to form an unstable resonator therebetween. The circular mirror disclosed therein has a compound radius of curvature which is difficult to fabricate.
Prior art resonators employ complex optical reflective surfaces which are difficult to manufacture or employ a single optical cavity in which the conditions for providing an output beam having both high power and good optical quality cannot be independently optimized. Also, prior art resonators adapted for accommodating gain media having a large volume require mode control resonators having a large separation between the optical components defining the mode control resonator to provide an output beam having good optical quality. Additionally, both the power level and diameter of the output beam cannot be varied during the operation of the resonator without affecting the resonator performance.